The present invention generally relates to the synthesis of graft, block-graft and star-shaped copolymers.
The living polymerization techniques have been used to synthesize well-defined graft copolymers (Rempp et. al., 1986, Adv. Polym. Sci., 86:145; Sawamoto, 1991, Int. J. Polym. Mater, 15:197; Pitsikalis et al., 1988, Adv. Polym. Sci., 135:1). However, the previous methods have the disadvantages of being slow, and not yielding relatively pure products.
The preparation techniques of graft copolymers by living polymerization can be classified in xe2x80x9cgrafting onto,xe2x80x9d xe2x80x9cgrafting from,xe2x80x9d and xe2x80x9cgrafting through.xe2x80x9d Among these methods, the first is most often employed. Because the backbone and the side chain polymers are prepared separately, both the precursors and the final graft copolymer can be characterized accurately. The xe2x80x9cgrafting ontoxe2x80x9d method is bas ed on a coupling reaction between the functional groups of the backbone and another living polymer. When the anionic polymerization technique is employed, the backbone polymer should possess numerous electrophilic groups. The most thoroughly studied systems are poly(methyl methacrylate) and partially functionalized polystyrene as backbone and polystyrene, poly(xcex1-methyl styrene) or poly(ethylene oxide) as side chains. However, in the coupling of the polystyryl anion with chloromethylated poly(styrene), or of the poly(xcex1-methyl styryl) anion with poly(methyl methacrylate), the reaction ceased after some branches were grafted to the backbone polymer. This occurs in spite of the presence of numerous remaining functional groups on the backbone and is due to the hindrance caused by the side chains already introduced. The reduction in the reactivity of the remaining functional groups leads to graft copolymers with an uncontrolled low degree of grafting.
The epoxy group possesses a higher reactivity for the polystyryl anion than the ester and chloromethyl groups. The coupling between the epoxy group and the anionic living polystyrene was employed in the preparation of a graft copolymer consisting of a polystyrene backbone and a polystyrene side chains, by reacting the living polystyrene with a poly(p-vinylstyrene oxide) backbone or its styrene copolymer; a high degree of grafting was thus attained. See, M. Takaki et al., Macromolecules, 10, 845 (1977). However, the backbone polymers were prepared by radical homo- or co-polymerization, and had to be carefully purified by several reprecipitations followed by freeze-drying under high vacuum. Even with such precautions, the impurities were not removed and usually caused the deactivation of a fraction of the living polymers. Because of the presence of deactivated homopolymers, it was difficult to obtain pure graft polymers.
The present invention solves this problem which is found in the other types of living polymerization processes.
The present invention provides a method for the preparation of graft, block, and star copolymers. The method involves in situ coupling reaction between a living anionic backbone polymer possessing epoxy side chains and other living anionic polymers. Compared to the previous coupling methods for the preparation of graft copolymers, the present method is simple, the coupling reaction is fast, pure copolymers are produced, and the molecular weight, graft number and grafting position can be controlled to a greater extent than in existing methods.